This application incorporates by reference Taiwan application Serial No. 90126086, filed Oct. 22, 2001.
1. Field of the Invention
The invention relates in general to the power supply circuit converting a DC voltage to a AC voltage, and more particularly to the power supply circuit for a cold-cathode fluorescent lamp.
2. Description of the Related Art
The LCD (Liquid Crystal Display) monitor is popular in these years because of being low in radiation, lightweight and compact. For example, portable electronic devices such as the notebook computers are equipped with LCDs for portable purposes.
The LCD panels can be classified into a reflective type and a transmissive type. The LCD panels of the transmissive type require back lighting. Cold-Cathode Fluorescent Lamp (CCFL) is commonly used as back lighting source, because it needs only simple control circuits and has the high power efficiency and longer life. The CCFL is started up by supplying a high AC voltage thereto. In a notebook computer, the high AC voltage is supplied by a power supply circuit, which converts the DC voltage outputted by the battery into the high AC voltage.
FIG. 1 is a diagram of a conventional power supply circuit 100 for the CCFL. The power supply circuit 100 is the Royer type circuit, which includes switches 104, 106, and a transformer 108. The power supply circuit 100 converts the DC voltage outputted by the DC voltage output circuit 102 into a high AC voltage for driving the CCFL 110. The transformer 108 is used for stepping up the voltage inputted thereto. The switches 104 and 106 are bipolar junction transistors (BJT). The collectors of the switches 104 and 106 are coupled to the two end nodes of the primary side of the transformer 108, respectively. The middle node of the primary side of the transformer 108 is coupled to the positive node of the DC voltage output circuit 102. The emitters of the switches 104 and 106 are coupled to the negative node of the DC voltage output circuit 102. The two nodes of the feedback circuit 112 of the secondary side of the transformer 108 are coupled to the bases of the switches 104 and 106, respectively. The bias resistance R1 is coupled between the positive node of the DC voltage output circuit 102 and the base of the switch 104. The CCFL 110 and the decoupling capacitor C1 are connected serially with the secondary side of the transformer 108.
FIG. 2A is the equivalent circuit diagram of the power supply circuit 100 while the switch 104 is on and the switch 106 is off. FIG. 2B is the equivalent circuit diagram of the power supply circuit 100 while the switch 104 is off and the switch 106 is on. The voltage outputted by the DC voltage output circuit 102 controls the on/off status of the switches 104 and 106, and the polarity of the primary side of the transformer 108 changes accordingly, as shown in FIGS. 2A and 2B. The polarity of voltage of the secondary side of the transformer 108 also changes according to that of the primary side. The transformer 108 steps up the AC voltage at the primary side and outputs the high AC voltage to the CCFL 110 via the decoupling capacitor C1 at the secondary side, according to the turn ratio of the primary side and the secondary side.
The main disadvantage of the power supply circuit 100 is the low power efficiency, which is about 70%xcx9c80%. Thus the usage time of the battery after each charge is reduced. The lifetime of the CCFL is also reduced. The transformer 108 has a complex structure that makes it expensive and difficult to manufacture.
FIG. 3 is a diagram of another power supply circuit 300 for the CCFL. The power supply circuit 300 includes switches 304 and 306, formed with MOSFETs, the capacitor C1 and a transformer 308. The switch 304 is an N-channel MOSFET, and the drain thereof is coupled to one node of the primary side of the transformer 308, and the other node of the primary side is coupled to the positive node of the DC voltage output circuit 302. The on/off statuses of the switch 304 and 306 are controlled by the switch control circuit 312. The negative node of the capacitor C1 is connected to the drain of the switch 306, and the positive node thereof is connected to both the drain of the switch 304 and one node of the primary side of the transformer 308. Two nodes of the diode D1 are connected to the drain and the source of the switch 304, respectively. And two nodes of the diode D2 are connected to the drain and the source of the switch 306, respectively. The diodes D1 and D2 are either the intrinsic diodes of the MOSFETs, or external diodes connected to the MOSFETs.
The operation of the power supply circuit 300 is described in FIGS. 4A to 4C. FIG. 4A is the equivalent circuit diagram of the power supply circuit 300 when the switch 304 is on and the switch 306 is off. The DC voltage output circuit 302 supplies a positive voltage to the primary side of the transformer 308, and the corresponding current flows from the DC voltage output circuit 302, to the transformer 308, and then to the switch 304. FIG. 4B is the equivalent circuit diagram of the power supply circuit 300 when the switches 304 and 306 are off. At this time, the voltage of the primary side of the transformer 308 is still positive, but the magnitude of the voltage thereof decreases with time. The current flows from the primary side of the transformer 308 to the capacitor C1 for energy preserving and charges the capacitor C1 to make the voltage thereof increases with time. FIG. 4C is the equivalent circuit diagram of the power supply circuit 300 when the switch 304 is off and the switch 306 is on. At this time, the capacitor C1 discharges and the voltage of the primary side of the transformer 308 is negative. By alternating the on and off status of the switches 304 and 306, the polarity of the voltage of the transformer 308 also alternates, as shown in FIGS. 4A to 4C. At the same time, the primary current I1 that flows through the primary side of the transformer 308, and the secondary current I2 that flows through the secondary side of the transformer 308 each also alternates the flow direction accordingly.
The disadvantage of the power supply circuit 300 is that the control mechanism is complex because three phases are required for the switch control circuit 312 to control the on/off status of the switches 304 and 306. Besides, the precise timing control of the on/off status of the switches 304 and 306 are required and thus the control mechanism is more complex.
FIG. 5 is another well-known diagram of the power supply circuit 500. The power supply circuit 500 includes the energy-preserving capacitor C1 coupled to the primary side of the transformer 512 in parallel, the energy-preserving inductor L1 coupled to the energy-preserving capacitor C1 and the primary side of the transformer 512, and four MOSFETs used as switches 504, 506, 508, and 510. The switch 504 is electrically connected to the positive node of the DC voltage output circuit 502, energy-preserving inductor L1 and the switch 506. The switch 508 is electrically connected to the positive node of the DC voltage output circuit 502, the primary side of the transformer 512, the capacitor C1 and the switch 510. The switch 506 is further connected to the switch 510.
The operation scheme is described in FIGS. 6Axcx9c6D. FIG. 6A is the equivalent circuit diagram of the power supply circuit 500 while the switch 504 and 510 are on, and the switch 506 and 508 are off. At this time, the DC voltage output circuit 502 charges the energy-preserving capacitor C1 and the energy-preserving inductor L1. The polarity of the primary side of the transformer 512 is positive, and the magnitude of the voltage thereof increases with time. The current flows from the energy-preserving inductor L1 to the primary side of the transformer 512. FIG. 6B is the equivalent circuit diagram of the power supply circuit 500 while the switch 506 and 510 are on, and the switch 504 and 508 are off. At this time, the capacitor C1 discharges, and the current flows form the capacitor C1 to the primary side of the transformer 512, the polarity of the voltage of the primary side is still positive, and the voltage of the primary side decreases with time. FIG. 6C is the equivalent circuit diagram of the power supply circuit 500 while the switch 506 and 508 are on, and the switch 504 and 510 are off. At this time, the DC voltage output circuit 502 charges the energy-preserving inductor L1 and the energy-preserving capacitor C1. The polarity of the primary side of the transformer 512 is negative, and the voltage thereof decreases with time. The direction of the current, flowing through the primary side, is different from that in the equivalent circuit shown in FIG. 6B. FIG. 6D is the equivalent circuit diagram of the power supply circuit 500 while the switch 506 and 510 are on, and the switch 504 and 508 are off. At this time, the capacitor C1 discharges, and the current flows from the capacitor C1 to the primary side of the transformer 512. The polarity of the voltage of the primary side is still negative, but the magnitude of the voltage of the primary side increases with time. Thus, the polarity of the voltage of the primary side of the transformer 512 alternates between positive and negative according to the alternative change of the on/off status of the switches 504, 406, 508, and 510. And the current I1 that flows through the primary side of the transformer 512 and the current I2 that flows through the secondary side of the transformer 512 also alternate directions accordingly as shown in FIGS. 6Axcx9c6D.
The disadvantage of the power supply circuit 500 is that the manufacture is complex because four switches are required, and the control mechanism is complex because the control mechanism needs to precisely control the on/off status of the switches 504, 506, 508, and 510 in four different phases.
It is therefore an object of the invention to provide an improved and simplified power supply circuit for the CCFL, which has the following advantages:
1. Manufacturing of the power supply circuit is easy.
2. Control mechanism is easy.
3. Power efficiency is good.
The invention achieves the above-identified objects by providing a power supply circuit. The power supply circuit for the CCFL is coupled to a DC (Direct Current) voltage output circuit and the CCFL. The DC voltage output circuit outputs a low DC voltage, and then the power supply circuit converts the low DC voltage to a high AC voltage for driving the CCFL. The power supply circuit includes a switch, a switch control circuit, a transformer, an energy-preserving unit, and a decoupling capacitor. The switch has a control node, a ground node, and a signal node. The switch control circuit is coupled to the control node, for outputting a control signal to control the on/off status of the switch. The transformer has a primary side and a secondary side. The primary side has the first node and the second node, and the secondary side has the third node and the fourth node. The first node is coupled to the DC voltage output circuit, the second node is coupled to the signal node of the switch. The energy-preserving unit is for preserving electrical energy. The energy-preserving unit has a fifth node and a sixth node. The fifth node is coupled to the first node of the primary side of the transformer and the DC voltage output circuit. The decoupling capacitor is coupled to the third node of the secondary side of the transformer for outputting the high AC voltage.
Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings.